From the July 1882 Parker Bros. catalog.
Quality U is a Grade 0 Twist Barrels 12-gauge with a straight grip. List Price $50
Quality T is a Grade 0 Twist Barrels 12-gauge with a pistol grip. List Price $55
Quality S is a Grade 0 Twist Barrels 10-gauge with a straight grip. List Price $55
Quality R is a Grade 0 Twist Barrels 10-gauge with a pistol grip. List Price $60
Quality Q is a Grade 1 English Twist Barrels 12-gauge with a straight grip. List Price $60
Quality P is a Grade 1 English Twist Barrels 12-gauge with a pistol grip. List Price $65
Quality O is a Grade 1 English Twist Barrels 10-gauge with a straight grip. List Price $65
Quality N is a Grade 1 English Twist Barrels 10-gauge with a pistol grip. List Price $70
Quality M is a Grade 1 Laminated Barrels 12-gauge with a straight grip. List Price $65
Quality L is a Grade 1 Laminated Barrels 12-gauge with a pistol grip. List Price $70
Quality K is a Grade 1 Laminated Barrels 10-gauge with a straight grip. List Price $70
Quality I is a Grade 1 Laminated Barrels 10-gauge with a pistol grip. List Price $75
In that I've never seen a Parker Bros. rib marked "English Twist" but have seen barrels marked "Twist" --
37865 04 TWIST.jpg
and "Stub Twist" --
STUB TWIST barrels 02.jpeg
How does this equate to Grade 0 and Grade 1?